Background prior art relating to touch sensing systems employing a plane of light can be found in U.S. Pat. No. 6,281,878 (Montellese), and in various later patents of Lumio/VKB Inc, such as U.S. Pat. No. 7,305,368, as well as in similar patents held by Canesta Inc, for example U.S. Pat. No. 6,710,770. Broadly speaking these systems project a fan-shaped plane of infrared (IR) light just above a displayed image and use a camera to detect the light scattered from this plane by a finger or other object reaching through to approach or touch the displayed image.
Further background prior art can be found in: WO01/93006; U.S. Pat. Nos. 6,650,318; 7,305,368; 7,084,857; 7,268,692; 7,417,681; 7,242,388 (US2007/222760); US2007/019103; WO01/93006; WO01/93182; WO2008/038275; US2006/187199; U.S. Pat. Nos. 6,614,422; 6,710,770 (US2002021287); U.S. Pat. Nos. 7,593,593; 7,599,561; 7,519,223; 7,394,459; 6,611,921; U.S. D 595,785; U.S. Pat. Nos. 6,690,357; 6,377,238; 5,767,842; WO2006/108443; WO2008/146098; U.S. Pat. No. 6,367,933 (WO00/21282); WO02/101443; U.S. Pat. Nos. 6,491,400; 7,379,619; US2004/0095315; U.S. Pat. Nos. 6,281,878; 6,031,519; GB2,343,023A; U.S. Pat. No. 4,384,201; DE 41 21 180A; and US2006/244720.
We have previously described techniques for improved touch sensitive holographic displays, in particular in our earlier patent applications: WO2010/073024; WO2010/073045; and WO2010/073047. In such systems the camera viewing the plane or fan of light detects a certain amount of ambient light as well as the light scattered by a user's fingertip. Where an infrared (IR) laser is used to generate the touch sensing plane of light the amount of ambient light can be reduced by employing an optical notch filter at the laser wavelength, but further suppression of ambient light is desirable. One technique to achieve this involves alternating “IR laser off” with “IR laser on” frames. Touch detection is then implemented on the difference of these frames to subtract out any ambient IR in the scene. However, this can cause problems: For example when a finger is moving so rapidly that it is in a different place in the laser off and laser on frames this can cause a false touch event.